The first slow-down comes from the Speed Table. At its core, the speed system gives a Spd-N character N actions per 12-second turn. Keeping track, segment by segment, of who can act according to the chart though is a pain, especially if different characters have different Spd stats and players want to optimize their choices based on who can or cannot act before their next phase. Why not just roll a d12 to see if you can act? Or a d6 for Spd stats under 6, and a guaranteed action plus an extra action (depending on a d6 roll) for Spd stats over 6?